corporate-wage-slave

This has got a lot better in the last year or two, and though it's not a generous connection then you should make it even if the BA arrival is 30 minutes late. MIA used to be dreadful most of the time and awful much of the time; these days it's probably better than JFK (etc) overall, particularly if GE. Just watch that the GE machines may be on the far side of the CBP area from where you walk in, but there are plenty of machines.

For the Flight Connection issue, it's true that no USA airport has a proper Flight Connection experience, though PHL runs close, and MIA actually isn't far behind. There is a dedicated connections channel into MIA right by the international arrivals exit, albeit via landside. It is clearly signposted for AA connections, it can also be used for BA connections though you do end up in the D concourse rather than E, and it's just to left of the Arrivals exit into landside. It consists of a few check-in desks and a small security channel away from the main D security. It doesn't have PreCheck, however they give you a card to indicate your have an expedited process, and so you can keep shoes on and leave laptops in the bag. You may need to reprint your AA boarding passes in AA.com, or use the App, to ensure that PreCheck appears on the boarding pass, and in any case you should put your Known Traveler number into the SABRE booking reference on AA.com. BA.com seating module will show the SABRE locator.

If it's not the end of the world that you get rebooked to the next flight in the event of delays, then a 90 minute connection seems ideal to me.